They know that there's something out there that has mass but they can't see it. So when you gather up all the information about stars and planets and all the stuff that's made out of ordinary stuff, that can only account for about four percent of the universe. And the rest of it seems to be in some form of invisible matter - just called dark matter.

You've no doubt heard of the placebo effect, however…

There's a kind of split in scientific opinion on this. There's very strong evidence that shows no effect of placebo whatsoever when you look at all the scientific literature. And yet on an individual case-by-case basis there does seem to be a lot of evidence that a placebo actually does have an effect. There's a classic study of Diazepam (which is Valium) that shows that it doesn't reduce your anxiety level unless you actually know you're taking it. When people get it in a hidden dose they say they're still just as anxious as they were.

If the placebo effect does work it seems to only work on things that are subjective like pain or anxiety.

It doesn't seem to have any effect on things like lung disease or cancer. So, the power of positive thinking in that respect doesn't seem to be able to help you.